Sources of combustion create exhaust gas that contains harmful pollutants including, but not limited to, particulate matter (PM), hydrocarbons (HC), nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), engine lubricating oil, and unburned fuel. To reduce the dangers of exhaust gas to human health, regulating agencies including the United States Environmental Protection Agency (EPA) and state agencies set maximum emission limits. Engine and boiler manufacturers and/or operators install emissions treatment systems to meet these increasingly stringent regulations. Emissions sources are categorized as either stationary sources or mobile sources.
Examples of mobile sources include, but are not limited to oceangoing vessels and locomotives. One of many examples of an emissions treatment system that travels with a mobile source is one that travels with an oceangoing vessel. Another example is an emissions treatment system that travels with locomotive(s) in railcars. An emissions treatment system for a mobile source may travel with the mobile source and/or connect to the mobile source when the mobile source is temporarily not travelling but continuing to generate emissions. Emissions treatment systems for mobile sources are more challenging due to the absence of a fixed connection to utilities such as water.
Many mobile sources that are stationary for a period of time but continue generating emissions. Thus, mobile emissions treatment systems are frequently connected to mobile sources while they are stationary. One example is a mobile emissions treatment system connected to an oceangoing vessel at berth in order to control emissions from the vessel's auxiliary generator(s) and/or boiler(s). Another example is a mobile emissions treatment system connected to a stopped or slow-moving locomotive in a railyard which continues to generate emissions. These are just some examples. Thus, mobile emissions treatment systems share many of the same challenges as emissions treatment systems that travel with mobile sources in the respect that there is no fixed connection to utilities such as water.
Emissions treatment systems that contain hot gas quenchers and/or wet scrubbers can waste thousands of gallons of water per day through evaporation. For wet scrubbers used in stationary sources, this may not be as much of an issue because there is typically a convenient and inexpensive connection to a water utility. However, emissions treatment systems for mobile sources that use wet scrubbers must carry thousands if not tens of thousands of gallons of water in order replace water lost to evaporation. Mobile sources with emissions treatment system or mobile emissions treatment system systems must store enough water on board in order to operate sufficiently long between water tank fill-ups. This can be impractical because of size of the water tanks, the weight of the water tanks, the cost of filling remote water tanks, and/or the logistical problems caused by frequent refilling of the tanks.
Typical emissions treatment systems may include the following elements that may contribute, either directly or indirectly to the evaporation of water into the exhaust gas:                a) Exhaust filters such as a Diesel particulate filter (DPF) to reduce PM        b) Selective catalytic reduction (SCR) to reduce gaseous emissions such as NOx        c) Heater(s)        d) Fans and/or blowers.        e) Gas coolers        f) Wet scrubbers to remove SO2         g) Electrostatic wet scrubbers to remove PM and SO2         
Some of these elements are described below:
Exhaust Filters
Exhaust temperatures from a source of combustion may reach a temperature of 540° C. to 650° C. Furthermore, sometimes supplemental heaters are used upstream or inside the exhaust filter or DPF in order to reach the operating temperatures required for the DPF.
One disadvantage of emissions treatment systems that use exhaust filters is their high operating temperature. When used upstream of a wet scrubber, for example, the high operating temperatures can cause a tremendous amount of water to be evaporated.
Selective Catalytic Reduction (SCR)
When an SCR is used in an emissions treatment system, they operate efficiently within an elevated temperature range which is typically between 200° C. and 315° C. In order cause the exhaust gas to be within this temperature range, heaters are sometimes used upstream of the emissions treatment system. This evaporated water is typically exhausted from the emissions treatment system and lost to the atmosphere.
One disadvantage of emissions treatment systems that use SCR's, is their elevated operating temperature. When used upstream of a wet scrubber, for example, the high operating temperatures of the SCR can cause a tremendous amount of water to be evaporated. This evaporated water is typically exhausted from the emissions treatment system and lost to the atmosphere.
Gas Coolers
Another element of some emissions treatment systems is a gas cooler. Gas coolers may take the form of hot gas quenchers or wet scrubbers, for example. Hot gas quenchers, are used to reduce the exhaust gas temperature prior to another process. These gas quenchers evaporate water which causes the exhaust gas to cool due to the latent heat of evaporation. The exhaust gas is cooled to near the water saturation temperature according to the pressure within the quencher. Another example of a gas cooler is a wet scrubber, which also cools the exhaust gas temperature through evaporation. The primary use for wet scrubbers, however, is to remove gaseous pollution such as SO2. Gas coolers such as, but not limited to, hot gas quenchers and/or wet scrubbers, suffer from a number of disadvantages:                (a) A disadvantage of gas coolers is they can cause the exhaust gas to become saturated with moisture.        (b) Another disadvantage of gas coolers is they can evaporate a relatively large amount of water. Any water that is not evaporated in a gas quencher and/or wet scrubber is typically returned to a sump where it can be reused. In some cases, the quencher and/or scrubber operate as part of a mobile system that typically has no direct connection to a water utility. In a mobile application, the water must be stored in a large tank which must be periodically filled. If the mobile system operates apart from a water utility, then water must be transported to the location of the mobile system. The replacement of this evaporated water can therefore become expensive.        (c) Yet another disadvantage of gas coolers is the saturated gas can cause visible plumes that exit from an exhaust pipe. Plumes can cause visibility problems, which can be dangerous. Plumes also make a negative impression on observers because plumes have the appearance of smoke. Therefore, observers may falsely get the impression that pollution is being emitted from the exhaust pipe even though it may only be water vapor. This mistaken negative impression can harm the reputation of a manufacturer and/or operator of the emissions treatment system.        (d) Yet another disadvantage of gas coolers is the saturated gas can cause problems downstream in the pipe or ducting. If the gas cools as it travels through the pipe or ducting, then the moisture will likely condense into liquid water. Water can accumulate in the pipe and ducting, possibly causing blockages or even failure of the ducting structure. Furthermore, water can also be carried by the exhaust gas as droplets. Water droplets can carry through the system and possibly damage downstream equipment. Furthermore, condensed moisture can lead to corrosion in the exhaust pipe or ducting. Thus, the condensed water caused by gas coolers can cause damage to the equipment, which can be expensive and cause downtime and delays.        (e) Yet another disadvantage of gas coolers is that condensed water produced can also interfere with accuracy of sensors that are normally placed in the pipe or ducting. Furthermore, condensed water can also interfere with emissions and/or flow measurements that are sometimes required by government regulators. Thus, condensed water gas coolers can cause faulty measurements, equipment malfunction, inaccurate test results, and/or downtime and delays.        
In view of the foregoing, there is a demand for a liquid conservation device for emissions treatment systems that use gas coolers without the aforementioned disadvantages.